State $\times$ Year Fixed Effects & \checkmark & \checkmark & & \\
State Fixed Effects & & & \checkmark & \checkmark \\
Geoclimatic Controls   &   \checkmark & \checkmark & \checkmark & \checkmark \\
Smooth Location Controls   &   \checkmark & \checkmark & \checkmark & \checkmark \\
Frontier Status & & \checkmark & & \\
Total Frontier Experience & & & & \checkmark \\
